Bolometers are energy detectors based upon a change in the resistance of materials (called bolometer elements) when exposed to a radiation flux. The bolometer elements have been made from metals or semiconductors. In metals, the resistance change is essentially due to variations in the carrier mobility, which typically decreases with temperature. Greater sensitivity can be obtained in high-resistivity semiconductor bolometer elements in which the free-carrier density is an exponential function of temperature, but thin film fabrication of semiconductor for bolometers is a difficult problem.
FIG. 1 provides a perspective view illustrating a three-level bolometer 1, disclosed in a copending commonly own application, U.S. Ser. Application No. 09/207,054, entitled "INFRARED BOLOMETER WITH AN ENHANCED STRUCTURAL STABILITY AND INTEGRITY" and FIG. 2 presents a schematic cross sectional view depicting the three-level bolometer 1 taken along A--A in FIG. 1. The bolometer 1 comprises an active matrix level 10, a support level 20, a pair of posts 40 and an absorption level 30.
The active matrix level 10 has a substrate 12 including an integrated circuit (not shown), a pair of connecting terminals 14 and a protective layer 16. Each of the connecting terminals 14 is electrically connected to the integrated circuit and the protective layer 16 covers the substrate 12.
The support level 20 includes a bridge 22 made of an insulating material and a pair of conduction lines 24 made of an electrically conducting material. The bridge 22 is provided with a pair of anchor portions 22a, a pair of leg portions 22b and an elevated portion 22c. Each of the anchor portions 22a is fixed to the active matrix level 10 and includes a via hole 26 through which one end of each of the conduction lines 24 is electrically connected to each of the connecting terminals 14 in the active matrix level 10, each of the leg portions 22b supports the elevated portion 22c on which the other end of each of the conduction lines 24 is electrically disconnected from each other. Additionally, the elevated portion 22c is formed to have a serpentine shape to minimize the thermal exchange between the active matrix level 10 and the absorption level 30.
The absorption level 30 is provided with a bolometer element 36 surrounded by an absorber 32, a reflective layer 34 formed at bottom of the absorber 32 and an infrared absorber coating 38 (hereinafter, "IR absorber coating") positioned on top of the absorber 32. The reflective layer 34 is made of a metal is used for returning the transmitted IR back to the square absorber 32. The IR absorber coating 38 is used for enhancing an absorption efficiency.
Each of the posts 40 is placed between the absorption level 30 and the support level 20, wherein a top portion of each of the posts 40 is attached to the center portion of the absorber 32 and a bottom portion thereof is attached on the elevated portion 22c of the bridge 22. Each of the posts 40 includes an electrical conduit 42 made of a metal and surrounded by an insulating material 44. Top end of the electrical conduit 42 is electrically connected to one end of the bolometer element 36 and bottom end thereof is electrically connected to the respective conduction line 24 of the supporting level 20, in such a way that both ends of the bolometer element 36 in the absorption level 30 are electrically connected to the integrated circuit of the active matrix level 10 through the electrical conduits 42, the conduction lines 24 and the connecting terminals 14.
When exposed to infrared radiation, the resistivity of the bolometer element changes, causing a current and a voltage to vary, accordingly. The varied current or voltage is amplified by the integrated circuit, in such a way that the amplified current or voltage is read out by detective circuit (not shown).
One of the major shortcomings of the above-described bolometer is a structural instability caused by the ways in which the stresses accumulated therein during the forming thereof are released. For example, as shown in FIG. 3, since the absorber 32 having a square shape is, at its center, supported by the posts 40, the stresses therein tend to be relieved at its respective corner portion in the direction indicated by arrows, resulting in deforming an entire configuration of the absorber 32, detrimentally affecting to the structural integrity of the infrared bolometer 1.